Compile with stable Rust

[sunchao]

One of major nightly feature we rely on: impl trait, has been stabilized in the latest 1.26 release. We should see if it's easy to remove the nightly dependency and move to stable.

Current dependencies:

• specialization (31844)
• std::arch::{x86, x86_64} (49664, 1.27)
• type_ascription (23416)
• rustc_private (27812)
• try_from (33417)